The present invention generally relates to a centre conductor for connecting an LC-filter with a connector, for example a coaxial connector fixed on a coaxial cable. The invention also relates to an LC-filter comprising at least one such centre conductor.
In LC-filter networks for filtering selected frequencies, for example when LC-filters are used in communication systems, for instance in radio base stations, it is a problem that mechanically joined electrical couplings between two components are likely to create a P-N-junction, thereby creating intermodulation interference problems Therefore, it is desirable to create a network with as few couplings as possible, especially friction couplings and brazed couplings, to reduce the intermodulation interference.
Therefore, it is desirable to invent an apparatus with as few couplings and parts as possible for connecting, for example, a standard coaxial cable to an LC-filter. However, so far such an apparatus still involves brazed couplings and parts that create undesired intermodulation interference.
U.S. Pat. No. 5,001,443-A discloses coaxial units for conducting microwaves to and from amplifying apparatus with splitter and combiner cavities. A coaxial coupling unit is fixed to a first plate member and has a tubular outer conductor and an inner conductor that are insulated from each other by a sleeve. The inner conductor has a lower pin part with a free end that has a frusto-conical exterior shape with slots, which extend upwards. The slots divide the pin into axially extending fingers, which are deflectable radially inwards. This enables the pin to be pressed into a hole in a second plate member. The inner conductor thus creates a friction coupling with the second plate member. The upper end of the inner conductor is shaped as a stub
The embodiment of the inner conductor disclosed in U.S. Pat. No. 5,001,443-A has features that are undesirable if the coaxial unit for some reason would be used in an LC-filter. A friction coupling of the described type demands small limits of tolerance during manufacturing of both the coupling unit and the hole in a receiving member of the LC-filter, since it is a friction coupling exclusively by press fit It is also very difficult to achieve a predetermined desired coupling force between the inner conductor and the receiving member. Furthermore, if the inner conductor is fixed to the coaxial unit, the conductor and the hole in the sleeve would require very small limits of tolerance for a secure press fit, or an extra arrangement, for keeping the inner conductor fixed in the coaxial unit. Of course this increases the manufacturing costs of the coaxial unit. If the inner conductor is not a fixed part of the coaxial unit, a detachment of a to the coaxial unit before connected coaxial cable may withdraw the inner conductor from the hole or decrease the contact surface between the hole and the inner connector. This reduces the reliability of proper contact when a coaxial cable is to be attached again, or creates a renewed assembly time to reinstall the coaxial unit in a proper way. Also, since the upper part of the inner conductor is shaped as a stub, the inner conductor does not fit a standard connector that is fixed to a coaxial cable and has a pin member projecting out from the connector. This kind of connector is common for LC-filters, such as cavity filters.
It is therefore a general object of the present invention to solve or reduce the above mentioned problems and disadvantages by providing an improved apparatus for connecting an LC-filter with a connector, in terms of intermodulation interference, contact area with the LC-filter, accurate fastening to the LC-filter, assembly time, manufacturing, storage costs and logistics costs.
It is also a general object of the present invention to provide an improved LC-filter in such a way that the LC-filter enables an improved connection with a connector, such as a coaxial connector fixed to a coaxial cable, by using the improved centre conductor and modifying the LC-filter for an improved fit with the centre conductor.
The present invention therefore provides a single-piece centre conductor for an LC-filter. Thereby, the centre conductor does not involve two or more parts that have to be connected, e.g. by brazing. This reduces intermodulation interference problems and saves assembly time. Furthermore, it saves storage costs as well as logistics costs since fewer parts have to be manufactured, registered and stored.
The centre conductor comprises a first end section with axially form confining fixing means, and a second end section with means for direct connection to connecting means of a connector, such as a coaxial connector fixed to a coaxial cable. With axially form confining fixing means are meant: fixing means that due to its form see to that the centre conductor substantially is not able to be axially removed in relation to the LC-filter when the centre conductor is mounted to the LC-filter and subjected to a tractive force. Thus when the first end section is mounted to the LC-filter, for example by engaging a partially threaded bore in the LC-filter, the fixing means substantially prevent axial movement of the centre conductor relative to the LC-filter when the centre conductor is subjected to a force substantially parallel with a longitudinal axis of the centre conductor. With that a problem with possible disconnection between the first end section and the LC-filter is avoided in case of disconnection of the coaxial cable from the LC-filter. The axially form confining fixing means also are allowed to have larger limits of tolerance than such a press fit as the one mentioned before in the discussion about U.S. Pat. No. 5,001,443-A. This makes the centre conductor easier to manufacture than if friction means were to be used.
The means at the second end section preferably comprise a cavity for engaging the connecting means of the connector, where the connecting means is a pin member. This enables the centre conductor to be directly connected to a pin member of a standard coaxial connector.
Suitably, the cavity is surrounded by at least two tongues, which are resiliently deflectable outwards from, and equiangularly distributed about, the longitudinal axis of the centre conductor. This provides easy and fast attachment and unattachment of the pin member to the centre conductor, as well as a press fit that secures a good contact between the centre conductor and the pin member.
In order to improve the assembly time and the convenience for the installer, the centre conductor comprises at least one tool receiving means for engaging a tool during mounting of the centre conductor to the LC-filter. The tool receiving means may comprise a cross section with a regular polygonal shape, such as a hexagon shape, which enables engagement with an ordinary socket wrench. Alternatively or in addition to the above mentioned polygonal shape, the tool receiving means may comprise at least one aperture, for example a through-hole or an exterior slot, for engagement with pins, clutches or other kinds of assembly tools.
Furthermore, the centre conductor preferably comprises a middle section with a larger cross sectional area than the first and second end sections. Suitably the middle section comprises a substantially flat ring-shaped surface, which is perpendicular to the longitudinal axis of the centre conductor and has a normal pointing outwards towards the first end section. This enables the ring-shaped surface to contact an opposite flat surface around the receiving bore of the LC-filter. Thus a larger contact area between the LC-filter and the centre conductor is created.
The form closing fixing means are preferably threads. This enables the centre conductor to be fixed in the bore of the LC-filter with a predetermined force, which easily is determined since sufficient torque meters for screw joints are widely used in assembly workshops. The threads also enables a type of cold welding between the ring shaped surface and the opposite flat surface of the LC-filter as the ring shaped surface is screwed into contact with the opposite flat surface. The type of cold welding further improves the contact between the centre conductor and the LC-filter.
The present invention further provides an LC-filter, which for instance is used in a radio base station, with at least one such a centre conductor as described above. Preferably the LC-filter is a cavity filter with an input resonator, comprising a bore having an outer mouth and wall, which comprises threads and engages the centre conductor, and a substantially flat surface that surrounds the outer mouth and is substantially perpendicular to a longitudinal axis of the bore. The LC-filter may also comprise an output resonator engaging a second centre conductor of the above described type.